The invention relates to a sensor, which uses the capacitive measuring principle, and a method for detecting an approaching dielectric medium, preferably for detecting a human body part, said sensor being adapted for use in an anti-pinching system, and comprising a capacitor and an evaluation electronic system, it being possible to measure the variation of the capacitance of the capacitor, which variation is caused by the dielectric medium.
Capacitive proximity sensors have been known for a long time from practical experience. They comprise specially designed capacitors, the electric stray field of which is influenced by approaching objects. Non-conductive objects, due to their increased relative permittivity in relation to the ambient air, lead to an increase in the capacitance of the sensor. The variation of the capacitance is dependent on the distance of the object from the sensor, the location of the object in relation to the sensor, the dimensions and relative permittivity of the object. The capacitance of the sensor must be determined for detecting an approaching object. All capacitance-measuring methods known from practical experience to those skilled in the art can be used for this purpose. In most cases, the sensor is a part of a tuned circuit, which is put out of tune by approaching objects or which becomes capable of oscillation only by the presence of an object in the stray field of the sensor, given the suitable dimensions of the tuned circuit. Specially designed proximity sensors can be used particularly advantageously in an anti-pinching system. An example of this is disclosed in the German laid-open patent specification DE 102 48 761 A1 (U.S. Patent Publication No. 2006/0139036).
Due to the high water content in the human body and the very high relative permittivity of water, a human body part in the stray field of a proximity sensor leads to a particularly high measuring effect. However, this helps detect not only the presence of a human body part but also water and/or moisture present in the field of the sensor. This leads to erroneous measurements, particularly when there is rain or fog.
This problem is solved in the patent specification DE 102 48 761 A1 mentioned by comparing the measurement results of several sensors of this type to each other and by adjusting the threshold value of the individual sensors, under the assumption of a uniform water/moisture distribution in the range of the combined sensors and the associated constant increase in the capacitance of all the sensors. However, it is not always possible to ensure the accuracy of this assumption. Furthermore, the use of several sensors with associated evaluation electronic systems results in high costs and the necessity for a mutual compensation of the sensors.
Other approaches known from practical experience provide for the use of additional compensation electrodes, which can attenuate the effects of water and/or moisture in the stray field of the sensor, given the suitable circuit connections and dimensions. However, in this case also, expensive compensation measures are required.
It is therefore an object of the present invention to design and refine a sensor and a method of the type mentioned in the introduction, for detecting an approaching dielectric medium, preferably for detecting a human body part, so as to use a simple construction and ensure a reliable measurement independently of environmental effects, particularly moisture and/or water.